Manufacturers of internal combustion engines develop engine control strategies to satisfy customer demands and meet various regulations for emissions and fuel economy. One such engine control strategy comprises operating an engine at an air/fuel ratio that is lean of stoichiometry to improve fuel economy and reduce greenhouse gas emissions. Such operation is possible using compression-ignition (diesel) and lean-burn spark-ignition engines. When an engine operates with lean (excess oxygen) air/fuel ratio, the resultant combustion temperature is lower, leading to decreased engine-out NOx emissions; However, commercial application of lean-operating engines is limited due to lack of effective methods to remove NOx under a lean exhaust condition. Thus, efficient reduction of nitrogen oxides (NOx=NO+NO2) from diesel and lean-burn gasoline exhaust is important to meet future emission standards and improve vehicle fuel economy.
Reduction of NOx emissions from an exhaust feedstream containing excess oxygen is a challenge for vehicle manufacturers. By way of example, it is estimated that compliance with Bin 5 regulations in the United States may require an aftertreatment system capable of 70-90% NOx conversion efficiency on the FTP (Federal Test Procedure) cycle based on currently anticipated engine-out NOx levels. For practical application, the conversion efficiency must be obtained at a low temperature operating range (e.g., 200-350° C.) occurring during the aforementioned FTP cycle and at a higher temperature operating range (e.g., 450-550° C.) occurring during a high speed test cycle (e.g., US06 federal test procedure).
Several potential aftertreatment systems have been proposed for vehicle applications. One approach comprises using an aftertreatment system including injecting a NOx reductant, e.g., urea, upstream of a urea-SCR catalyst, to reduce NOx to N2. Use of urea as a reductant necessitates a urea distribution infrastructure and an on-vehicle monitoring system for this secondary fluid, and may have potential problems in cold weather climates due to the relatively high freezing point (−12° C.) of the urea solution. NOx storage catalysts typically require large catalyst volumes, large amounts of platinum-group metals and low sulfur fuel for efficient storage operation. Such systems require periodic catalyst regeneration involving fuel injection to generate high exhaust gas temperatures and injection of reductants to regenerate the storage material of the catalyst.
There is a need for an effective apparatus and method to introduce a reductant into an exhaust gas feedstream over a broad range of operating conditions, for use on vehicles and other applications of internal combustion engines.